This invention relates to damping systems, and more particularly to a passive damping system using a combination of piezoelectric and viscoelastic elements.
Adding damping, or energy dissipation, has been and is a critical problem for many structures, including but not limited to large space platform truss structures, launch vehicle shrouds, automobile structures, and civil engineering structures. Traditionally, many structures have been treated with layers of viscoelastic materials (both constrained and unconstrained) to increase the overall damping of the structure. A viscoelastic material (VEM) is one which is viscous but which also exhibits certain elastic properties such as the ability to store energy of deformation. In such a material, the application of a stress gives rise to a strain that approaches its equilibrium value slowly. A classical traditional approach is to sandwich a layer of VEM between a structural member and a rigid constraining layer, the rigid constraining layer being more rigid than the VEM layer. When a force is applied to the structural member, from a vibration, a change in motion of the structure, or the like, the member deforms, with the portion of the VEM layer adjacent to the member deforming along with it. The purpose of the rigid constraining layer is to fix the portion of the VEM layer adjacent to it, thereby setting up an increased shear strain in the VEM layer. As a result, more energy is dissipated per cycle of vibration and damping of the vibration occurs more rapidly than would be the case without the constraining layer, thereby increasing the damping capacity of the system.
More recently, embedded piezoelectric ceramics with some associated electronics have been used to dissipate structural vibrations in beams. An example of this type of system is disclosed in U.S. Pat. No. 4,626,730. In that system a piezoelectric film is applied either directly to a structural member or atop an intermediate VEM layer. The structural member has on it a plurality of strain sensors, which are connected to a signal generator and amplifier circuit. The strain sensors, in turn, are connected to the piezoelectric film. In operation, when the strain sensors detect a deformation of the structural member, the signal generator is activated to signal the piezoelectric film. The voltage from the signal generator induces a strain in the piezoelectric film, which in turn induces distributed shearing displacement across the entire viscoelastic layer, thereby increasing damping capacity.
The problem with the above discussed systems is that the classical approach, employing a rigid constraining layer, often does not produce an adequate level of damping capacity. The second system, employing a piezoelectric film, requires a fully active circuit to obtain adequate damping response, adding a great deal of expense and complexity. What is needed, therefore, is a passive, simple, self-contained system which provides the level of damping response obtainable by an active system such as that discussed above.